1. Field of the Invention
This invention relates to optical polarization devices, and more particularly to non-absorbing, normal-incidence, reflective polarizing optical elements employing a multilayer stack of optically birefringent thin film layers obtainable by oblique vapor deposition in vacuum.
2. Background Information
The prior art includes several well-known light polarizing components. For example, prism polarizers, commonly known as Nicol and Glan polarizers, are based on double refraction of birefringent crystals--usually calcite. These components utilize polarization-selective total internal reflection at the interfaces of two matched prisms made of birefringent crystals. With these polarizers, one of the linearly polarized light components of incident light is removed by total internal reflection, while the orthogonal component is transmitted. These prism polarizes off an extinction ratio of higher than 100,000 and can cover a relatively wide spectral range. For example, calcite polarizers nominally have a useful spectral range of 0.35-2.3 .mu.m. However, these crystal polarizers are limited in size by the scarcity of calcite of good optical quality. In addition, they tend to be expensive, bulky and extremely sensitive to angle of incidence.
A variation of the Glan type polarizer is the cube polarizer disclosed in U.S. Pat. No. 2,403,731 issued to MacNeille. This polarizer consists of two glass prisms cemented together along their hypotenuses. The diagonal face of one of the prisms is coated with a multilayer dielectric film stack designed to reflect one linear polarization component and transmit the orthogonal component. Although much less costly than crystalline prism polarizers, MacNeille cubes typically exhibit a lower extinction ratio of about 1,000 and can operate over a wavelength band of only about 300 nm.
Additional significant prior art includes dichroic polarizers, such as those commonly referred to as "Polaroid.RTM.", sheet polarizers. These polarizers are based on polarization-selective absorption, or dichroism, which are first discovered by Biot circa 1815. Modern sheet polarizers are based on an invention by Edwin Land circa 1928. This technology is reviewed by the inventor in Journal of Optical Society America, 41, 957(1951). These widely used sheet polarizers can typically provide an extinction ratio of 1,000 and are of low cost. High extinction ratio is possible but is achieved at expense of transmittance efficiency. Because they are constructed of organic polymers, they possess severe inherent limitations, i.e. they are limited to low optical power handling capability, limited to use at low operating temperature and restricted to use in the visible spectral range.
Another example of a dichroic polarizer recently introduced by Corning Glass Works of Corning, N.Y., is sold under the trademark Polarcor.TM.. This polarizer is made of glass which contains elongated submicroscopic silver particles aligned in the same direction. The silver particles preferentially absorb light polarized along the long axis of the silver particles while transmitting light of the orthogonal polarization. These glass-based filters are durable and may typically withstand a temperature up to 400.degree. C.; however, they exhibit relatively narrow bandwidth and are limited in operating wavelength range from approximately 0.633 to 2.1 .mu.m.
A further example of a dichroic filter includes a metal film obliquely deposited on a glass substrate as disclosed in U.S. Pat. No. 5,122,907 (hereinafter, the '907 patent) issued to Slocum. This dichroic film is made up of microscopic prolate metal spheroids all aligned in the same direction on the glass surface. A polarizer of this general type, commercially available from Melles Griot Company of Irvine, Calif., for example, operates in the near-infrared spectral range and has a relatively low transmittance of only about 20%.
Other important prior art includes polarizers based on organic cholesteric liquid crystalline (CLC) materials. This art has been taught, for example, by Lee and Jacobs, in a paper published in Journal of Applied Physics, Vol. 68, No. 274 (1990). Cholesteric liquid crystalline thin films, with molecular alignment in the so-called planar texture, exhibit wavelength- and circular polarization-selective reflectance over a spectral range which is material-dependent. Such films may operate satisfactorily as circular polarizers in many applications, but because they are organic materials, they absorb strongly in the UV spectral range, limiting their operating range. Moreover, the operating temperature is limited to about 140.degree. C. These films may be used as a transmissive linear polarizers in conjunction with a quarter-wave plate which converts the polarization of transmitted light from circular to linear.
Recently, Iwatsuka et al disclosed polarizing devices based on diffraction grating based on obliquely deposited thin film in U.S. Pat. No. 5,245,471. One of the limitations of such devices is that the 0-th diffracted light beam remains unpolarized.
Thus, a need exists for an improved polarizing optics method and apparatus for polarizing light in a manner which achieves a substantial improvement in efficiency and power handling capacity.